Add SmartDashboard::GetEntry function in C++ (#2064)

Existed in Java already.

wpilibc/src/main/native/cpp/smartdashboard/SmartDashboard.cpp

@@ -81,6 +81,10 @@ void SmartDashboard::Delete(wpi::StringRef key) {
   Singleton::GetInstance().table->Delete(key);
 }
 
+nt::NetworkTableEntry SmartDashboard::GetEntry(wpi::StringRef key) {
+  return Singleton::GetInstance().table->GetEntry(key);
+}
+
 void SmartDashboard::PutData(wpi::StringRef key, Sendable* data) {
   if (data == nullptr) {
     wpi_setGlobalWPIErrorWithContext(NullParameter, "value");

wpilibc/src/main/native/include/frc/smartdashboard/SmartDashboard.h

@@ -11,6 +11,7 @@
 #include <string>
 #include <vector>
 
+#include <networktables/NetworkTableEntry.h>
 #include <networktables/NetworkTableValue.h>
 
 #include "frc/ErrorBase.h"
@@ -96,6 +97,16 @@ class SmartDashboard : public ErrorBase,
    */
   static void Delete(wpi::StringRef key);
 
+  /**
+   * Returns an NT Entry mapping to the specified key
+   *
+   * This is useful if an entry is used often, or is read and then modified.
+   *
+   * @param key the key
+   * @return    the entry for the key
+   */
+  static nt::NetworkTableEntry GetEntry(wpi::StringRef key);
+
   /**
    * Maps the specified key to the specified value in this table.
    *